This invention concerns lubricating compositions having improved resistance to oxidation. Another aspect of the invention relates to antioxidant synergists and their incorporation in lubricating compositions to improve resistance to oxidation thereof.
Lubricating oils, greases and similar oleaginous materials are used under conditions which contribute to their breakdown during normal service. The severe high temperature operating conditions of modern engines accelerate deterioration of lubricants due to oxidation. Oxidative deterioration is accompanied by formation of gum, sludge and acids which may cause corrosion of the engine as well as chemical breakdown of the lubricant.
Additives performing as antioxidants are often corrosive themselves or break down during normal use into corrosive substances which result in severe corrosive attack on metals and premature replacement of machinery.
It is known that certain alkyl substituted benzotriazole derivatives possess metal passivating properties as described in British Patent Specifications Nos. 1,511,593 and 1,466,558. Furthermore, U.S. Pat. No. 4,701,273 teaches that methylated 1-[di(2-ethylhexyl)aminomethyl]benzotriazole displays a synergistic action with respect to metal deactivation and oxidation inhibition when used in conjunction with a select group of antioxidants.
Surprisingly, it has been discovered that a phenyl derivative of tolutriazole shows unexpected synergistic effect with respect to antioxidant activity in lubricants when used in conjunction with one of several antioxidants.